System.FormatException when using function with custom handler as roles source of SWA

#### What language does your question apply to? (e.g. C#, JavaScript, Java, All)
C#, Go

#### Question

I'm using an Azure Function inside a Static Web App. The function is used to get the user roles in `staticwebapp.config.json`:
```
"auth": {
"rolesSource": "/api/GetRoles",
...
}
```
The function itself has a custom handler defined in host.json. The custom handler is implemented in Go:
```
"customHandler": {
"description": {
"defaultExecutablePath": "./build/server",
"workingDirectory": "",
"arguments": []
},
"enableForwardingHttpRequest" : true
},
...
```
The function is called after authentication but a System.FormatException is thrown:
```
Exception while executing function:
Functions.GetRoles The format of value 'text/plain; charset=utf-8, application/json' is invalid.
```
So the `content-type` header is clearly invalid but I don't no why.

According to Azure App Insights the exception originates from `Microsoft.Azure.WebJobs.Script.Extensions.HttpRequestExtensions.ToHttpRequestMessage`, so the exception is actually thrown before my custom handler for the function is called.

Not sure if this is a bug or I'm doing something wrong but any hints would be appreciated.

Start at Microsoft.Azure.WebJobs.Script.Extensions.HttpRequestExtensions.ToHttpRequestMessage and compare the request from the rolesSource in staticwebapp.config.json with the custom handler settings in host.json. Reproduce the /api/GetRoles call and trace how the content-type value is converted; done means the function executes without System.FormatException and returns the roles response.

Written by the indexing model from the issue text.